IF ALAN ALDA were cheese, he'd be warm brie. The Hush Puppies liberal, Walter Mondale act-alike and left-wing sex symbol returns to the screen for the first time since 1981's "Four Seasons" with his comfortable romantic comedy "Sweet Liberty." He stars, writes and directs with his trademark amiability. The movie succeeds, primarily thanks to his well-cast costars.

Alda plays a college professor who wrote a book on the American Revolution, a best-seller that Hollywood hacks are turning into a teen sex farce. When the moviemakers arrive in town to start filming, he salvages his book's historical integrity by stroking the egos of Michele Pfeiffer and Michael Caine, the movie's stars.

The slight central satire, in which Alda the moviemaker tweeks the cheeks of the industry, is lightly amusing, but extraneous subplots obscure the big picture. One concerns the professor's precious affair with a lovable literary colleague (Lise Hilboldt) and another focuses on his relationship with his dear old mother (Lillian Gish is delightful as a senile coquette).

Still, it's great to see Gish up there, her charisma shining on. And Pfeiffer, at the other end of time, is impossibly beautiful in one of her best roles as a tough-talking screen goddess, a mercenary method actress who is transformed into a demure colonial dame on the set. "I must remember to avoid her in street clothes," mutters Saul Rubinek, long-suffering as the hotshot director.

Rubinek and Bob Hoskins (as an obsequious screenwriter) enliven the pace, which ambles when Alda's out on his own proving his virility with Pfeiffer or Hilboldt, who find him enticing despite love handles, blah personality and boxer shorts. Caine is even randier as an incorrigible philanderer, an eccentric matinee idol who insists on doing all his own stunts. Dangerous and debonair, Caine plays it child-like and makes us like him in spite of his great failings and foolhardiness.

After meeting all these imaginative, fun-loving fantasy makers, the professor learns to be freer with the facts, to put accuracy in perspective. "Sweet Liberty" is never liberating, never exhilarating. It's too mild-mannered, too well-behaved. But if it's you're a brie eater, indulge.